Benefits of VCM technique in Ka-band

The growing market of Earth Observation satellites, pushed by the scientific, institutional and commercial requests, is actually going to congest the allocated X-Band taking into account that EO systems, providing innovative and more demanding services and using more and more sophisticated sensing techniques, require for higher downlink data rates. Without going towards advanced techniques as dual-polarization, the bound of the X-Band have been found lower than 600 Mbps, resulting in a “bottle-neck” for future remote sensing grow capabilities. Therefore, wider Ka Band represents the alternative to improve the performances of future EES missions in terms of interference minimization, congestion risks reduction and data rate increase; unfortunately, at a given link availability – usually 99.5% - atmospheric absorptions experienced in Ka-Band highly exceed what measured in X-Band and results in a very high dynamic between clear and rain sky condition. Recent outcomes in modulation and coding techniques have provided to system designers innovative techniques and means to adapt the transmission chain performances to the link condition through on-board provision of several coding schemes characterised by different spectral efficiencies. Such techniques - ACM (Adaptive Coding Modulation) and VCM (Variable Coding Modulation) - have been implemented in the MHOMS modulator; they can be used by system designer to ensure a constant link margin by tuning the information data rate through the selection of the most appropriate coding scheme, according to the actual link conditions. VCM techniques described in this work propose both a dynamic technique that simply adapts the modulation & coding schemes in accordance to on-ground measured parameters and a “prediction based” technique that optimizes the transmission parameters according to the local statistics of atmospheric attenuation. The benefits of the usage of such techniques will be shown through the comparison with respect to the CCM (Constant Coding Modulation) case.